Comprehensive evaluation of zeolite/marine alga nanocomposite in the removal of waste dye from industrial wastewater

A systematic study integrating laboratory, analytical, and case study field trial was conducted to figure out the effective adsorbent that could be used for the removal of Congo red (CR) dye from industrial wastewater effluent. The ability of the zeolite (Z) to adsorb CR dye from aqueous solutions was evaluated after it was modified by the Cystoseira compressa algae (CC) (Egyptian marine algae). Zeolite, CC algae were combined together in order to form the new composite zeolite/algae composite (ZCC) using wet impregnation technique and then characterized by the aid of different techniques. A noticeable enhancement in the adsorption capacity of newly synthesized ZCC was observed if compared to Z and CC, particularly at low CR concentrations. The batch style experiment was selected to figure out the impact of various experimental conditions on the adsorption behavior of different adsorbents. Moreover, isotherms and kinetics were estimated. According to the experimental results, the newly synthesized ZCC composite might be applied optimistically as an adsorbent for eliminating anionic dye molecules from industrial wastewater at low dye concentration. The dye adsorption on Z and ZCC followed the Langmuir isotherm, while that of CC followed the Freundlich isotherm. The dye adsorption kinetics on ZCC, CC, and Z were agreed with Elovich, intra-particle, and pseudo-second-order kinetic models, correspondingly. Adsorption mechanisms were also assessed using Weber's intraparticle diffusion model. Finally, field tests showed that the newly synthesized sorbent has a 98.5% efficient in eliminating dyes from industrial wastewater, authorizing the foundation for a recent eco-friendly adsorbent that facilitate industrial wastewater reuse.


Preparation of zeolite/CC algae composite (ZCC). Wet hydrothermal impregnation technique was
used to fabricate ZCC composite 34,35 . Here equal weights of zeolite and dry CC algae were combined in small amount of distilled H 2 O and magnetically stirred for 60 min at 500 rpm, followed by 60 min in an ultrasonic bath to form a paste and to attain a homogeneous impregnation of CC in the surface of Z support. The obtained paste was dried in a vacuum oven at 60 °C for 24 h. The Z, CC, and ZCC composite were characterized by Fourier transformer infrared (FTIR) spectrometer, X-ray diffractometer (XRD), and Scanning electronic microscopy (SEM).
Preparation of the adsorbate. In this work, the adsorbate was chosen to be CR, a common anionic dye.
CR is the sodium salt of 3,3′-([1,1′-biphenyl]-4,4′-diyl)bis(4-aminonaphthalene-1-sulfonic acid)with a formula: C 32 H 22 N 6 Na 2 O 6 S 2 as shown in Fig. S1 (Supplementary data). CR stock solution of concentration of 1000 ppm was prepared by dissolving 1 g of CR dye in 1 L of distilled H 2 O. CR solutions were prepared at various concentrations using the diluting method. The pH was adjusted to 3, 5, 7, and 10 by adding 0.1 M NaOH or 0.1 M HCl solution to the solution.
The CR solution volume was fixed at 20 mL in each experiment. By tracking the absorption peak, the CR concentration variation was evaluated with a Perkin Elmer Lambda 950 UV/Vis/NIR spectrophotometer. Z, CC and ZCC reusability were investigated for four cycles where 20 mg of each adsorbent was added to 20 mL of the CR dye with initial concentration 10 mg/L and the experiments were conducted for 480 min at fixed conditions of temperature and pH (25 °C and pH7). After each run; the adsorbent was removed from the solution, washed with distilled water and prepared for the next run.
The CR removal % as well as the CR elimination amounts after a period t (q t ) and at equilibrium (q e ) were calculated using Eqs. 1 and 2 36,37 .
(1) CR dye removal % = (C 0 − C i ) C 0 × 100 Adsorption isotherms. The adsorption isotherms of the fabricated Z, CC, and ZCC nanocomposite for the studied CR were explained using Freundlich, Langmuir, and Temkin models [38][39][40] . In supplementary data more details regarding the adsorption isotherms equations and their parameters are explained. Equation 3 can be applied to define the Langmuir isotherm's degree of favorability for equilibrium data utilizing the dimensionless separation factor's value (R L ) 41 .
where C max denotes the CR dye maximum initial concentration.
Adsorption kinetics and mechanisms. Several adsorption mechanisms and kinetic models such as internal particle diffusion, pseudo-first and second-order models, and Elekovech kinetic models are investigated to determine the adsorption mechanisms and kinetics associated with CR adsorption on Z, CC and ZCC 4,5,[42][43][44][45][46] .
Supplementary data presented more details about adsorption kinetics equations and their parameters. The average value of all adsorption results was measured in triplicate. OriginPro 2018 statistical functions were used to obtain regression coefficient (R 2 ) values for various kinetic and isothermal models.
Field experiments. The newly manufactured adsorbent was evaluated as an effective eco-friendly adsorbent that could be used on a large scale to get rid of industrial waste dye from industrial wastewater. In this regard, wastewater samples containing waste dyes were delivered from a garment dyeing plant in Beni-Suef, Egypt. The wastewater samples obtained were used without further purification or dilution. The best adsorbent system was selected based on experimental results.

Results and discussion
Adsorbents characterizations. SEM characterization. Figure 1 shows the SEM images of Z, CC, and ZCC adsorbent. Figure 1A, the SEM image of zeolite, showing flake like structure with different particle sizes and rough surface with porous cavities on the surface which appears clearly in the SEM figures of natural Zeolite. Moreover, the coincidence between SEM and XRD data appears clearly in the crystallinity of zeolite samples revealed in SEM images. SEM images of CC alga, Fig. 1B, reveals the presence of cell wall and different porous structure. The SEM image of CC, reveals a less porous surface that affects its surface area which in turn affects its adsorption capacity. The roughness appearance on CC surface indicates the incorporation of different functional groups, amine, carboxylic groups, and alkyl groups within the pores of the cell wall of CC algae. Finally, modification of the zeolite surface with CC algae, the pores on the zeolite surface are covered with the CC particles as shown in the SEM image of the ZCC composite. These particles are self-assembled to show a rough surface from the agglomerated particles. SEM images of ZCC nanocomposite Fig. 1C confirm the formation of new type of structure as one can see the roughness as well as crystalline structure which indicates the incorporation of CC algae within the structure of zeolite.
Factors affecting the adsorption process. Effect of initial dye concentration. The amount of CR removed by adsorption is highly dependent on the starting CR concentrations. The variations in the elimination percent and the amount of CR adsorbed using Z, CC, and ZCC adsorbents at different initial concentrations versus time were demonstrated in Fig. 3a-f. Throughout the first stage of the adsorption process, the dye elimination percentages and the adsorption capacities are very high. Their growth slows down until equilibrium is reached. The presence of a large number of active sites found on the surface of the adsorbent can explain the rapid rate of removal at the beginning of the reaction. As the time procced, the active sites are completely occupied by CR molecules 58 . This results in repulsive interactions between CR molecules in the attractive regions of the adsorbent and those in the bulk liquid phase 37 . Consequently, the fraction of CR released decreases as the initial CR concentration increases. Over all the tested    www.nature.com/scientificreports/ Adsorbent dosage effect. At the optimal adsorbent dosage for maximum efficiency, The influence of the adsorbent dosage on the CR removal% was estimated with respect to adsorption cost. This is shown graphically in Fig. 4a, where the sorbent dosage varied from 0.01 to 0.05 g. From Fig. 4a, we notice that, for all adsorbents, the dye removal% increases by rising the sorbent dosage from 0.01 to 0.05 g; in the case of Z adsorbent, it increased from 46.15 to 64.61%, increased from 57.14 to 83.93% for CC adsorbent and from 84.44 to 98.00% for ZCC adsorbent. Increasing the number of hot spots by increasing the adsorbent mass could be the reason for which this observation could be accredited 3,4,37 . It was observed that significant jumps in removal occurred as the adsorbent amount of ZCC increased from 0.01 to 0.02 g and that of Z increased from 0.01 to 0.03 g. In the case of ZCC and Z, the variation of CR % removal was slightly reduced by increasing the sorbent dose above 0.02 g and 0.03 g, respectively. This may be due to the "shielding effect" that occurs when the amount of adsorbent increases. Due to the increase of adsorbent amount and the reduction of spaces between them, a dense layer is formed on the surface of the adsorbent. The active site is hidden from the CR molecule by the formation of a dense layer. Furthermore, CR molecules compete for a limited number of accessible active sites due to the overlap of Z and ZCC. Aggregation or agglomeration at high doses of Z and ZCC increases the diffusion path length of CR adsorption, thereby reducing the adsorption rate 43,60-62 .
Influence of pH. pH is considered one of the most essential parameters influencing the dye removal capacity of adsorbent in wastewater. The adsorption efficacy is affected by the pH of the solution, as the pH changes affect the ionization degree of the adsorbate molecules as well as the surface characteristics of the adsorbent 63 . In the pH range between pH 3 and pH 10, the pH influence on the CR removal percentage by Z, CC and ZCC adsorbents was studied as shown in Fig. 4b Figure 4b shows that when the pH of the solution is 7, the CR adsorption capacity of Z, CC, and ZCC reached their maximum limit. This www.nature.com/scientificreports/ may be due to the interaction between the CR and adsorbent is more prominent than the interaction between the adsorbent with OH − ions in the solution 64 . At lower pH levels, the positive charge on the solution interface growths, and the Z, CC, and ZCC surfaces appear to be positively charged. However, due to the protonation of the CR molecules, CR in the solution tends to be neutral. This situation leads to a reduction in the adsorption of anionic CR as presented at pH 5 65 . With increasing pH value, the positive charge on the solution interface reduces and CR becomes negatively charged by the interaction with OHions. Hence, positively charged Z, CC, and ZCC have a reasonable interaction with negatively charged CR molecules or with OHions. Therefore, when pH exceeds7, CR adsorption decreases 73 .
Influence of temperature. The temperature effect is refered as a considerable physicochemical factor in the adsorption study, due to the variation caused in the adsorption capacities of the adsorbents 66 . A temperature series was applied at temperature ranges 25, 40, 50, 60, 70, 80, and 90 °C, all data are presented in Fig. 4c. A reverse relationship was noticed between temperature and dye removal percent. This relationship could be ascribed by the fact that adsorption force brokendown upon exposure to temperature, this force is responsible for dye molecule adsorption on the adsorbent surface 75 . Moreover, damaging and weakening of the active sites between the adsorbent's active binding sites and the adsorbate species 3,76,77 . Consequently, the optimal temperature for CR adsorption onto all tested adsorbents was selected to be located between 25 and 30 °C. With the temperature, the percentage of CR removal decreases, indicating exothermic adsorption process.
Reusability test. All adsorbents under investigation (Z, CC, and ZCC) were subjected to a reusability experiment which was repeated for 4 cycles using identical adsorbent doses. The data illustrated in Fig. 4d  Adsorption isotherms. All data were fitted to Langmuir, Freundlich, and Temkin models using the statistical significance of correlation coefficient (R 2 ) for nonlinear fitting of Ce versus q e . Table 1 presents all values related to of Q o , K L , K F , n, K T , B, and R 2 , these values were computed from the nonlinear fitting of the plots in Fig. 5. From the data revealed in Table, CR adsorption on Z and ZCC adsorbents follows the Langmuir model with the best R 2 value. Consequently, a multilayer adsorption of CR molecules occured at the active sites on the surface of the adsorbent under investigation. At these active sites there are unequally available heterogeneous sites, each with varied adsorption energy and interacting molecules. The values of R 2 obtained from the fitting to Langmuir isotherms in case of Z and ZCC adsorbents were 0.9837 and 0.9737 at 25 °C, respectively. The R L value is less than unity, which indicates that the adsorption of CR in that studied case is valuable 68 .
On the other hand, the adsorption isotherm of CR onto CC followed the Freundlich model. Consequently, a single layer adsorption process occued on the active sites of the CC adsorbent, and no reaction occurred between the adsorbed CR molecules. The R 2 value was recored to be 0.9584 for the Freundlich isotherms of CC adsorbent, www.nature.com/scientificreports/ also the R L value is less than unity, indicating that the CR adsorption in the investigated situation has considerable valuability 68 . Moreover, The values of (1/n) in the Freundlich isotherm model in the adsorption process of CR dye on the CC adsorbent were less than unity, which indicaded that adsorption was valuble, and heterogenisity of the surface, with fewer interactions between adsorbed ions. Meanwhile, It refers to the CR adsorption occured by multi-molecular and multi-anchorage adsorption mechanisms, respectively 69,70 . Adsorption kinetics. In order to study the most favourable kinetic model, the process of the adsorption of CR on the surface of Z, CC, and ZCC adsorbents was followed at varying initial dye concentrations. t vs qt was used and plotted in Fig. 6 in order to show the nonlinear forms of the first-order, second-order, and Elovich kinetic models. After that, each of kinetics and statistical parameters; k 1 , k 2 , q e , β, α, and R 2 ; were obtained and all the data were presented in Table 2. The nonlinear regression values of the studied concentration range and examined models which presented in Table 2 indicated that adsorption of CR onto Z surface was fitted by the first-order kinetics at higher concentrations (15,20, and 25 ppm). Which was confirmed and supported by the good approximation between the estimated q e and experimental q exp . Adsorption of CR onto Z surface was fitted by Elovich kinetics at CR concentrations of 5 and 10 ppm indicating that adsorbent surfaces are not energetically homogeneous. While, in case of ZCC, the Elovich kinetics found to be more suitable indicating that adsorbent surfaces are not energetically homogeneous which also confirmed by the higher R 2 values 71 . Finally, the adsorption of CR molecules onto the surface of CC adsorbent follows first-order kinetics at lower concentration, while it follows Elovich kinetics at lower concentration.
Sorption mechanism. Weber's Intra-particle diffusions was found to be more favourable in the fitting of the experimental findings in order to better understand the mechanisms and rate-controlling processes that  www.nature.com/scientificreports/ impact directly on adsorption kinetics. The nonlinear fitting of q t versus Ce, Fig. S2 (Supplementary data), supports the applicability of the intra-particle diffusion model's. Meanwhile, each of slopes and intercepts of the plots are utilized to compute K 3 and I value, Table 3. The values of I ≠ 0 indicated that the intra-particle model may not be the only rate-controlling process in identifying the adsorption process' kinetics 72 . Moreover, the boundary layer effect is clearly reflected by the intercept obtained in Fig. S2. The relation is a reverse on as bigger the intercept, corresponding to more surface adsorption contributes to the rate control step 72 .

Batch experiments and comparison with other adsorbents.
Depending on the experimental findings obtained during laboratory tests, well-selected optimized parameters for the newly-synthesized ZCC adsorbent were applied in a real field experiment in order to find out the applicability of our new adsorbent in the real industrial process. The parameters including 0.02 g of the adsorbent, room temperature, a non-changed pH of the wastewater containing waste dye, and the contact time was adjusted at 420 min. The industrial wastewater was subjected to scanning of wavelengths which revealed the presence of different wavelengths corresponding to different dyes. After the experiments were reached to a completion, absorbance was recorded using the same scanning device to estimate the removal percent of the dyes from the industrial wastewater. The promising data revealed that the ZCC nanoadsorbent succeded to catch up the different dyes from industrial wastewater with a 98.5% efficiency, which confirmed the foundation of new environmentally benign adsorbents that could be applied to reuse the industrial wastewater. Table 4 compares the adsorption capacity, q m , and dye removal percent of different studied adsorbents reported in the past work with those of Z, CC, and ZCC for CR dye adsorption. It appears that q m values vary broadly for different adsorbents [73][74][75][76][77] . The results stated that Z, CC, and ZCC displayed reasonable capacities for CR dye adsorption from aqueous solution relative to other adsorbent materials 73-77 . Hypothesis for organic adsorption of dye over different studied adsorbents. Figures 7,8 and 9 reveal interactions between zeolite, Cystoseira compressa, and ZCC composite surface and CR dye molecules. These interactions summarized in hydrogen bonds between oxygen and amine group in CR dye molecule as well as electrostatic interaction between negative and positive charges. This hypothesis confirmed by IR results.

Conclusions
A hydrothermal technique was selected for the preparation of of novel alga/zeolite composite ZCC from Z and CC. In order to remove CR dye from industrial wastewater, ZCC was introduced as a novel adsorbent at various conditions. The experimental results revealed that the lower the initial CR concentration, the greater the removal percent of CR to be improved also the removal rate was high during the adsorption experiment's early stages. The removal% increased with increasing Z, CC, and ZCC dosage from 0.01 to 0.05 g and it highly affected by temperature. For all adsorbents, the CR removal% increased with changing the initial pH from 3 to 10 and the supreme adsorption occurs at pH 7. The reducibility test for Z, CC, and ZCC adsorbents showed that all the adsorbents were not favored for reuse for the CR removal. The isotherms of CR adsorption onto Z, CC, and ZCC show that Z and ZCC adsorbents track the Langmuir isotherm models while CC follows the Freundlich isotherm models. Moreover, the CR adsorption onto Z was well-fitted to the second-order kinetics, while CC follows the intra-particle kinetics models. Furthermore, the CR adsorption onto the surface of ZCC was well fitted with Elovich kinetics models. More optimistic, the field experiments showed promising results as the ZCC nanoadsorbent succeeded to catch up the different dyes from industrial wastewater with a 98.5% efficiency, which confirmed the new born of novel environmentally benign adsorbents that could be applied to www.nature.com/scientificreports/ reuse the industrial wastewater. Finally, the supposed mechanisms for the adsorption of CR dye over adsorbents under investigation were in line with the data obtained from the IR charts and all the interactions summarized in hydrogen bonds between oxygen and amine group in CR dye molecule as well as electrostatic interaction between negative and positive charges.

Data availability
The datasets used and/or analysed during the current study available from the corresponding author on reasonable request.